Swing Check vs. dual check vs. double check valve

Hi and thanks in advance for the great info I've already gotten off this forum!

I recently had a steam shower generator installed. At the rough in inspection, the inspector said that I needed a dual check valve in the line that connects the autoflush drain from the generator, which is gravity fed not pumped, to the DWV. The way the line is plumbed is from the generator with 1/2" copper to the PVC shower drain
pipe before the shower trap.

The plumber insists that I don't need a dual check valve, and installed a swing check valve. The plumber also thinks that a dual check valve may cause my steam generator to malfunction. I've called the maker of the steam unit and technical support there was not much help.

Can somebody please tell me what to do. Who's right - the plumber or the inspector? Can you explain how each of these valves work so that I can weigh in during the next rough-in inspection?

Yes, it's true I'll let the inspector win in the end. However, I'd like to know the correct thing to do.

The unit does say install according to manufacturers instructions, however it doesn't mention valves in the autoflush line at all. It leaves all the plumbing up to a licensed plumber. I have two licensed plumbers who disagree on the best way to handle this. What is at stake is the operation of the somewhat expensive steam unit. Yes it's covered under warranty, but not if it is installed incorrectly. I'm also not one of those people who says, "oh I'll just sue somebody later if it's done wrong."

It seems that both these valves will prevent DWV water from backing up into the generator, which is what everyone is worried about. One valve does it with a swinging arm that is forced to close under flow in the wrong direction. No problems with water flowing in the line in the correct direction. As I understand it, the dual check is spring operated, by what looks like a drop in pressure in the autoflush line. However, the autoflush line is not ever under pressure as the potable supply line is. The air pressure in the autoflush is that of whatever vents the line. The water pressure, when water is allowed to drain, is rho*g*h + ambient pressure. What plumber #1 is worried about is that the dual check might close and stay closed causing the unit not to drain or start to drain and then close. Is that even possible?

Can anybody explain the dual check better or enlighten me on the pressure that causes the dual check to operate or what might cause my steam unit to malfunction? If I'm totally missing the point, I'd appreciate an explanation of the correct thinking.

The dual check has an integral air gap which may be why he wants that particular valve. If we saw how the autoflush is plumbed it might make it more logical. It is spring loaded so it will take a certain amount of pressure to open it.

Thanks! I've attached my quick sketch of the plumbing I have seen with my own eyes. I know that pipe size and angles are important to plumbers, but this is just a sketch and doesn't represent that stuff. I you need more info about that stuff I can write it.

When we are talking about backflow, I don't think a single swing check is ever an approved solution. I would consult a licensed backflow prevention company if you want better info, because hopefully they have attended "Backflow Univ"

You could use something like a Watts #7 dual spring check. They have diagrams on their web site. Another option would be to create an air gap. It might be possible to pipe this auto flush line into the shower just below where the steam line enters the shower.

The autoflush MAY operate while there is still steam pressure in the unit, in which case it will be adequate to operate a double check valve. The few I have worked on have the drain going outside into a planter area, because you are NOT supposed to discharge "superhot" water directly into the drain system, and DEFINITELY NOT into an open shower drain where hot water under pressure could splash up and burn the user. But a better way, if it can be done depending on what is behind the wall would be an indirect connection using an air gap, similar to a washing machine drain.

You could use something like a Watts #7 dual spring check. They have diagrams on their web site. Another option would be to create an air gap. It might be possible to pipe this auto flush line into the shower just below where the steam line enters the shower.

Click to expand...

A cross connection between potable water and a sewer can not be directly connected. This would require an air gap unless the appliance already has one.